Between July 1990 and April 1993, several research contracts were negotiated between Centre de
recherches minérales (CRM) of Ministère de 1’Energie et des Ressources du Québec and McGill
University and École Polytechnique to participate in the Waite Amulet Covers Project. Other technical
and financial partners in the project were Noranda Technology Centre (NTC), on behalf of
Noranda Mine& Inc., and Canada Centre for Minera1 and Energy Technology (CANMET).
Overall project management was provided by NTC. The project was conducted under the auspices
of the MEND (Mine Environment Neutral Drainage) program. The principal objective was to design,
construct and evaluate the effectiveness of soi1 covers and a plastic or geomembrane caver in
reducing acid generation in reactive mine tailings.
The objectives of the CRM-University contracts were to fùlfil specific technical scopes of work
defined under the project. In particular, the Geotechnical Research Centre (GRC) of M&il1 University
was requested to provide design and pre-construction geotechnical characterization of
caver materials and tailings. Parameters that were measured included baseline grain size distribution,
compaction parameters and moisture drainage and retention characteristics. The GRC mandate
was extended during the second and third years of the project to include an assessment of the
effects of fieeze-thaw on the hydraulic conductivity and moisture retention characteristics, the effect
of acid leaching on the stability of caver materials and the long ter-m stability of a high density
polyethelene (HDPE) used as a cover.
The Geotechnical Group at École Polytechnique was contracted to measure the uncovered tailings
hydraulic properties and conduct flow modelling. The purpose of the modelling was to confirm the
water balance on covered and uncovered tailings.
The Department of Geological Sciences, M&il1 University, investigated the possible effects of
sulphide oxidation on the composition of sulphur-bearing gases such as COS, CS, and SO,.
The GRC studies found that the varved clay soi1 used as the impermeable caver in the composite
caver system consisted of about 80% clay and 20% silt size materials. Compaction parameters,
obtained from modified Proctor compaction, were 23.5 % for the optimum moisture content and
1.66 Mg/m3 for the maximum dry density. Moisture retention data indicated that clay cari withstand
significant moisture losses and remain saturated at suctions of up to 10 m of water.
Two cases of water availability were investigated during laboratory freeze-thaw studies on the clay.
The first adopted a positive water head to assess maximum water damage to the clay caver and the
second used a negative water head to simulate field condition in the autumn. The results indicated
that the integrity of the clay was reduced more (the increase in hydraulic conductivity, K, was
greater) under positive head of water. A maximum increase of 2.5 orders of magnitude was observed
in K (that is, from 1 X 10mc9m /s to 5 X 10s7c m/s) alter one freeze-thaw cycle. This was
attributed to: 1) aggregation of clay particles; 2) chemical interactions between the generated acidic
solutions and heavy metals fi-om the tailings, and the clay residence pore fluid, causing a reduction
of the thickness of the double layer and an increase in the tendency of flocculation of the clay particles
(hence, an increase in permeability); 3) reduction in the buffer-mg capacity of the clay due to
reduced pH values. The K remained essentially constant in subsequent fieeze-thaw cycles. Under
negative water head, the integrity of the clay was only slightly reduced (K increased by only 0.5
order of magnitude) due to: 1) limited amount of water intake during freezing; 2) limited chemical
interaction; however, the residence pore fluid cations in the clay were reduced as a result of further
leaching. The thickness of the double layer increases as does the tendency for clay dispersion,
which in turn reduces the permeability of the clay caver to water. The negative water head used in
the study, nevertheless, was much greater than the negative head at the air entry value of the clay
obtained from soi1 suction experiments. Thus the clay was unsaturated. The results suggest that the
case of a positive water head would be more applicable to field conditions. Thus physical structure
of the clay layer would require monitor-mg.
The final part of the GRC contract evaluated the compatibility of the 80 mil HDPE as a fùnction of
cyclic freeze-thaw and acidic leachate. The HDPE did not experience any changes in its physical
and chemical properties within the experimental time frame. The moisture drainage characteristics
of the clay were greatly intluenced by leaching and cyclic freeze-thaw, due to aggregation and,
hence, significant pore space readjustment. When the clay soi1 was subjected to leaching, air permeability
decreased by l-2 orders of magnitude and oxygen concentration dropped to about zero.
It was postulated that the difference in oxygen content, between samples saturated with distilled
water and those with acidic leachate, probably occurred due to the difference in oxygen solubility
in both liquids.
The laboratory results at École Polytechnique indicated that the Waite Amulet tailings vary considerably
in gradation and hydraulic properties over short distances. In the central part of the tailings
area, specific gravities ranged from 3.21 at a depth of 4.90 m to 3.55 at 5.60 m. The average particle
size represented by D,, (the particle size at which 10% of the tailings are finer) is about 0.01
mm. The tailings in this area are sandy silts, with non-plastic clay-size particles ranging from 2% to
10%.
Laboratory and field (piezometer) permeability tests gave hydraulic conductivity values in the range
of 1.3-2.0 x 10” crn/s for tailings located below the test plot area. Field permeability tests gave values
in the range of 1.0-3.0 x 10S5c m/s for the clay caver material outside the test plot (testing was
not allowed in the test plot).
The variable-head test results in six piezometers of the test plot area have shown that two of them
were hydraulically short-circuited: therefore the measurements for head, gradient, and water quality
parameters are questionable. The installation of the four other piezometers was considered successfùl.
These piezometers indicated a vertical downward seepage in the tailings with a gradient
close to 12%. Other piezometers close to the test plot area were not tested for hydraulic conductivity
and potential hydraulic short-circuiting. However their water levels helped with the computer
flow modelling work. Interestingly, several piezometers have shown that there was an inversion
of the vertical seepage (upward to downward) between May and June.
In the flow modelling work, École Polytechnique used two hydraulic conductivity values of 10m5
and 10-’ cm/s for the clay caver, and computer generated (not measured) curves for undrained
properties of tailings and clay. The K value of 10e5c rn/s was measured outside the test plot where
the clay had been compacted and deliberately lefi without any protective layer (Sand and gravel) to
evaluate fieeze-thaw effects. The initial K value for the clay was measured at 10-’ cm/s by NTC at
the end of construction. Field measurements made by NTC in the second year did not indicate any
change in K for this clay covered by 0.30 m of sand and gravel. It is therefore possible, based on
the GRC laboratory tests, that freeze-thaw had increased the K value of the unprotected clay by
two orders of magnitude afier one winter cycle.
Other parameters used in the flow modelling were the efficient infiltration of either 200 mm/y
(NTC estimate) or 500 mm/y (Polytechnique estimate), and the KH value of either 6 x 10mo4 r 2 x
10” crn/s for the tailings. The hydraulic conductivity anisotropy ratio for the tailings was used as a
variable with the objective to find which ratio gave the best results with the hydraulic head values
measured below the test plots.
The use of a K value of 1O mo5r 1@ ’ crn/s for the clay caver in the flow mode1 (École
Polytechnique) has not changed significantly the vertical infiltration of water into the tailings,
mainly because most of the water appeared to laterally bypass the test plot. TO match the registered
hydraulic head values, it was necessary to use an anisotropy ratio K&v of 50 to 280 with an
average value of 80. École Polytechnique has confirmed the results of its computer flow mode1
with a study of water mean armual budget in vertical and horizontal directions. The average anisotropy
value of 80 corresponds roughly to the estimated value of 100 that was obtained in a previous
study by NTC for tailings located in the south-end of the impoundment.
Later flow modelling conducted by NTC (as reported in Volume 1) used a K value of 10-’ crn/s for
the clay caver. Measured material properties for the tailings and the clay and the incorporation of
the geomembrane lining on the caver slopes was also taken into account. The results gave more
realistic percolation rates that were similar to lysimeter data.
The research conducted by the Department of Geological Sciences, M&il1 University, aimed at
improving the understanding of the role of sulphur-bearing gases during oxidation of sulphidic tailings.
The consisted of field sampling of tailings pore gas for analysis of CO, and sulphur bearing
gases (COS, CS,, H,S and SO,). It was postulated that the effectiveness of the covers in reducing
sulphide oxidation could be evaluated by examining pore gas concentrations over time. One of the
fimctions of the composite covers was to provide an impermeable barrier to meteoric waters and
therefore minimize the production of acid waters. A quantitative analysis of pore gas was, therefore,
considered a more comprehensive and sensitive indication of the extent and rates of sulphide
oxidation and hence caver performance. In addition to the pore gas analysis, a limited evaluation of
the geochemical evolution of water samples collected from covered and uncovered test plots was
also conducted.
Analysis of sulphur-bearing gases did not yield any H2S and SO,. Previous studies by other researchers
had indicated that SO, was detectable only over completely dry sulphide minerals. Given
relatively high solubilities of H,S and SO, in water and the relatively humid conditions at Waite
Amulet, it was concluded that any H,S or SO, produced by sulphide oxidation would likely be dissolved
in water.
Stable isotope analysis for 6% and 6180 in CO, intended to identifl the source of CO, did not produce
conclusive results. It was, however, inferred that sources of CO, could include (1) oxidation
of organic matter, (2) dissolution of carbonate minerals in tailings, (3) dissolution of carbonate
minerals in limestone added during revegetation, and (4) trapped air.
